In continuous inkjet printheads, ink is continuously jetted from each nozzle, with each inkjet breaking off to create streams of drops. A portion of the created drops are selected to strike the print media while the remaining drops are made to strike a catcher, which returns the ink from the non-print drops to the fluid reservoir. One common type of catcher, known as a Coanda catcher, has a drop impact face against which the non-impact drops strike. The ink from the impacting drops flows along the face of the catcher and around a curved surface to enter the fluid removal channel of the catcher, from which the fluid is returned to the ink reservoir.
U.S. Pat. No. 3,936,135 (Duffield) discloses a Coanda catcher, in which the non-impact drops impact a face of the catcher, and the ink flows around the Coanda radius to enter the fluid removal channel. It is taught that by maintaining the vacuum, used for extracting the ink from the fluid removal channel below a certain critical level, a stable meniscus can be established at the entrance of the fluid removal channel thereby preventing air from being drawn into the fluid removal channel. As indicated in U.S. Pat. No. 4,035,811 (Paranjpe), the catcher arrangement of U.S. Pat. No. 3,936,135 is able to handle only moderate ink flow rates. EP 0 805 039 (Loyd) discloses a Coanda catcher having a fluid return channel designed to limit the amount of air ingested into the fluid return channel while providing a wider operating range of vacuum for evacuation of the ink than the U.S. Pat. No. 3,936,135 catcher.
For printheads having arrays of nozzles, the fluid removal channel must extend the length of the nozzle array. It is necessary to effectively remove ink from the fluid removal channel down the entire length of the fluid removal channel. U.S. Pat. No. 3,777,307 (Duffield) provided ink removal ports at each end of the catcher to avoid an excessive vacuum gradient within the catcher. U.S. Pat. No. 4,268,836 (Huliba et al.) describes a catcher in which the fluid removal channel of the catcher is divided down the length of the nozzle array into segments by flow partitions. Each segment has a separate ink removal port through which ink is removed from that segment to further improve the uniformity of the vacuum within the catcher. U.S. Pat. No. 6,187,212 (Simon et al.) segments the fluid removal channel into a number of flow channel branches, in which the lengths and widths of the different branches are selected to ensure approximately uniform flow impedances for each of the parallel branches and therefore provide more uniform fluid extraction across the width of the entrance of the fluid removal channel.
While these catcher designs are effective in removing ink across the width of the entrance of the fluid removal channel under normal conditions, ink extraction problems can occur when there are large differences in printed ink coverage across the width of a printhead. There remains a need to improve the extraction of ink from the catcher across the width of the printhead.